The present invention relates to apparatus and methods for analyzing and characterizing airborne particulate matter and more particularly to such apparatus and methods operating in real time.
This invention was developed with the use of funding provided in part by the combination of funding from Westinghouse Savannah River Technology Center pursuant to contract no. 3-30-1905-xxxx-32-4716 and the National Science Foundation pursuant to grant DMR-9727667.
The development of analytical methods for the characterization of airborne particulate matter has become an area of increasing activity over the last 15 years. The driving forces for these investigations lie across many fields of application, including environmental health and safety, atmospheric sciences, clean room quality control, and battlefield/non-proliferation monitoring.
Perhaps the greatest impetus for the development of new apparatus and methods of particle characterization has been the evolution of new air quality standards currently underway in the United States. Specifically, the U.S. Environmental Protection Agency (EPA) has proposed the PM2.5 standard for airborne particulate matter (hereafter xe2x80x9cthe PM2.5 standardxe2x80x9d). The PM2.5 standard limits the density of particles of less than 2.5 micrometers in diameter to a value of 15 xcexcg/m3 on an annual basis. The PM2.5 standard does not include any parameters that refer to the chemical composition of airborne particles, only their size distribution/density (2).1 The majority of the fields of application mentioned above rely on both particle classification based on size distribution as well as chemical composition (i.e., higher levels of information are required). As the pathogenic effects of particle composition become more apparent (which may in fact be driven through the development of improved analytical methods), it is easy to envision that more comprehensive regulations will evolve.
The numbers within the parentheses refer to the numbered endnotes at the end of the application. 
The characterization of airborne particulate matter is usually classified according to the sample system from which the chemical information is desired; batch or single particle. Given the wide range of possible sample types and information requirements, it is clear that no single method will be applicable in all cases. In fact, the classification of batch or single particle fields of application are an effective way to address capabilities.
In a batch sample system, particle size distributions and gross composition information are usually the intended goals. This type of analysis is most often applied in process monitoring situations where particle size distribution is often the most relevant piece of information.
In the case of single particle analysis, particle size and composition (elemental or molecular) are determined in order to assess the xe2x80x9cchemistryxe2x80x9d of a given system. These evaluations seek to define size/composition relationships, determine the distribution of species within/on a single or individual particle, and study chemical reactions at a particle""s surface. Beyond providing basic size/composition information, the requirements placed upon analytical instruments used in particle characterization include aspects of sample size, sample preparation/processing, analytical time frame, portability/remote monitoring capabilities, and instrument cost and complexity.
The most significant advances over the last decade in the analysis of airborne particulate matter have occurred in the area of single particle analysis (3-16). Some of these analytical methods involve collection of particles on inert filter supports for subsequent analysis by microbeam techniques. Others involve direct, real time sampling/analysis of individual particles by instruments which may be taken out into the field.
Van Grieken et al. Analyst, volume 120, pages 681-692 (1995) have reviewed the application of charged particle microbeam methods (e.g., EPMA, PIXE, SIMS) for the characterization of individual, collected particles. While these methods are quite powerful, particularly when used in tandem, the acts of particle collection, transport, and analyses in high vacuum environments present a number of drawbacks including poor temporal resolution, questions of representation, and possible loss of volatile analytes.
Real time single particle analysis methods often involve laser-induced vaporization/excitation/ionization and atomic emission or mass spectrometric detection. In the realm of elemental analysis, laser induced breakdown spectroscopy (LIBS) (4, 5) and laser excited atomic fluorescence (LEAF) (6) following thermal dissociation provide individual particle information. Hahn (5) used the 1064 nm fundamental output of a Nd:YAG laser to vaporize desolvated particles produced with a conventional solution nebulizer. Use of 1 xcexcm diameter Fe-doped polymer beads permitted the establishment of emission intensity/particle size relationships. Implicit in any quantification scheme of this sort is the summation of the responses of all species present in each particle. This of course requires a priori knowledge of the sample composition.
Panne and co-workers (6) diverted one-half of the flow from a nebulizer/desolvation system through a differential mobility particle sizer (DMPS) and the other half through the path of a vaporization/excitation laser. Temporal resolution of Pb atomic fluorescence from the background plasma emission permitted very sensitive detection (PbLOD=47 ng/m3). Low analytical duty cycles (limited by laser repetition rate) and single-element operation of the system were acknowledged as limitations, though the high level of selectivity and possibility for miniaturization were seen as positive features.
Direct (vacuum) inlets are versatile means of introducing ambient or collected particles. In the majority of such systems, differentially pumped momentum separators (often called particle beam interfaces) provide the means for performing analysis by methods requiring vacuum environments (i.e., mass spectrometry) and optionally identification/analysis of single particles in real-time (7). The research groups of Prather (8-11), Johnston (12-14), and Ramsey (15-16) have each made unique contributions to the field.
Prather and co-workers (9-11) have described the use of aerodynamic particle sizing using a dual-laser triggering system followed by laser vaporization/ionization and time-of-flight mass spectrometry (TOF-MS). One possible implementation of this time-of-flight mass spectrometry device is described in U.S. Pat. No. 5,681,752 to Prather. The production of both positive and negative ion species has been used to advantage in gaining comprehensive information from single aerosol particles.
Johnston (12-14) has used the intensity of scattered laser radiation as a measure of particle size and as a trigger for subsequent laser vaporization/ionization and TOF-MS analysis. One possible implementation of this time-of-flight mass spectrometry device is described in U.S. Pat. No. 5,565,677 to Wexler et al.
Ramsey and co-workers (15, 16) have exploited the ability of quadrupole ion traps to operate in modes which either trap charged particles or perform mass analysis of laser-produced ions. In the former mode of operation, charged particles can be effectively levitated within the three dimensional trap. Use of the ion trap as a mass analyzer provides higher levels of chemical information than TOF-MS as collision-induced dissociation (MS/MS) of isolated ions can be performed. Similarly, Davis and co-workers have used electrodynamic traps as a means of isolating charged particles at atmospheric pressure for interrogation by Raman spectroscopy (17).
Chemical analysis of batch-type (not single particle) particulate samples most often involves collection of samples via directed flow through a quartz fiber filter having pore sizes on the order of 1 xcexcm (18-23). Optical scattering or differential mobility particle sizing can be accomplished prior to deposition on the filter. Very different from the case of single particle analysis, batch chemical analysis is very seldom performed in real-time or on-site (remotely). Accordingly, issues of sample turnaround time, loss of temporal resolution, and possible sample loss or contamination are amplified.
Immobilized particles can be analyzed non-destructively via x-ray fluorescence (XRF) or particle induced x-ray emission (PIXE) (18). X-ray fluorescence analysis involves relatively simple instrumentation, is highly automated, and provides high sample throughput. Insensitivity to low Z elements and matrix effects due to differences in particle morphology can be limiting in some cases. Spectrochemical analysis by inductively coupled plasma atomic emission and mass spectrometries (ICP-AES/MS) has been applied to collected particulate matter in a number of fashions (19-24). Acid dissolution followed by solution nebulization is straightforward from the point of view of calibration and matrix normalization (19, 20). Small collected sample masses and general difficulties in achieving quantitative dissolution make this approach susceptible to contamination and challenge available detection limits. Laser ablation (LA) directly from the filter surface is an attractive approach to sample introduction for ICP-AES/MS analysis (21-23). Tanaka and co-workers (21) have demonstrated the use of standard solutions deposited on filter substrates as a very powerful means of quantification for LA-ICP-MS.
Collection of particulates via electrostatic precipitation has also been shown to provide a convenient means of presenting samples to atmospheric pressure plasmas (24). Bitterli et al. (24) have used a hollow graphite collector which can in turn be placed in an electrothermal vaporizer assembly in a manner similar to graphite furnace atomizers used for sample introduction to ICP-MS.
As a final example of filter collection for subsequent plasma source analysis, a commercially available plasma system (PT-1000, Yokagawa Electric Corporation, Tokyo) uses pneumatic transport to sweep particulate matter to a microwave-induced plasma (MIP) sustained with helium as the discharge gas (25, 26). Particles entering the plasma are thermally dissociated, and the emission from up to four analyte elements is monitored by separate monochromators. Particle size distributions are extrapolated from the total of the analyte emission responses based on a diameter-cubed relationship and the assumption that the particles are spherical in shape.
Related to the work described here is the application of a low pressure, glow discharge (GD) as an atomization and ionization source for collected particulate matter. Van Grieken and co-workers (27) mounted a metal target at the base of an impactor apparatus to collect airborne particles. The target was then mounted in the ion volume of the VG9000 glow discharge mass spectrometer system. Because the particle samples are electrically nonconductive in nature, a portion of the metal target was exposed to the plasma region to initiate the discharge such that the powders were sputtered from the surface, the constituent elements ionized within the negative glow, and then identified by their isotopic abundances. The collected particles were distributed in an approximately 2 millimeter diameter, cone-shaped mound of 50 to 90 micrometers height. The sample analysis times were on the order of 30 to 60 minutes, at which point the mass spectra were composed only of the species present in the target metal. Limits of detection were estimated to be in the low to sub-ppm level relative to the total sample mass. However, approximate masses of the sample sizes were not provided.
It is important to recognize that, while there are a plethora of applications wherein real-time particle sizing is performed on flowing streams, there is a scarcity of methodologies which accomplish elemental (much less molecular) species analysis in the continuous, batch-type mode of operation.
It is a principal object of the present invention to provide apparatus and methods employing low pressure plasmas for sampling, analyzing and characterizing particulate matter, both airborne and collected.
It is also a principal object of the present invention to provide apparatus and methods employing technology that employs a device for producing a low pressure plasma for sampling, analyzing and characterizing of particulate matter, both airborne and collected.
It is also a principal object of the present invention to provide apparatus and methods employing glow discharge technology for analyzing and characterizing of particulate matter (both airborne and collected) in the batch-type mode of operation as opposed to single particle methods.
It is another principal object of the present invention to provide apparatus and methods employing glow discharge technology for elemental species analysis of airborne particulate matter in the batch-type mode of operation.
It is a further principal object of the present invention to provide apparatus and methods employing glow discharge technology for molecular species analysis of airborne particulate matter in the batch-type mode of operation.
It is still another principal object of the present invention to provide apparatus and methods employing glow discharge technology for direct, remote monitoring of airborne particulate matter both in real time and particles collected in situ.
It is yet another principal object of the present invention to provide apparatus and methods employing glow discharge technology for direct, remote exhaust stack monitoring of airborne particulate matter both in real time and particles collected in situ.
It is a still further principal object of the present invention to provide apparatus and methods capable of a batch-type mode of operation and employing glow discharge technology for analyzing and characterizing of particulate matter as well as the gaseous components in which the particulate matter is entrained.
It is a still further principal object of the present invention to provide apparatus and methods for analyzing particulate matter in a particle beam (PB) subjected to low-power laser scattering to effect particle size analysis and introduced into low pressure (glow discharge) plasma sources for subsequent real time analysis by atomic emission and mass spectrometry.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.
To achieve the objects and in accordance with the purpose of the invention, as embodied and broadly described herein, the apparatus and methods of the present invention implement a particle beam (PB) sampling scheme for the introduction of particulate matter into low pressure (glow discharge) plasma sources for subsequent atomic emission and mass spectrometry chemical analysis in real time, whether the particles are provided in a continuous stream during the analysis or are collected (in situ or ex situ) and analyzed periodically upon obtaining a suitable number of particles to be analyzed.
The apparatus and methods of the present invention include a device for producing a low pressure plasma (e.g., a glow discharge unit), a momentum separator (also known as a particle beam interface), and a conduit.
The device for producing a low pressure plasma has a particle input port configured and disposed for receiving particles to be analyzed. The device provides the energy that ionizes and excites the particles and has at least one sampling region where ionized and/or excited particles are to be analyzed. In the glow discharge embodiment of the device for producing a low pressure plasma, particles enter the source volume and impinge on the heated walls (200 to 1000xc2x0 C.) of the source volume and are flash vaporized. The resultant atoms/molecules are subjected to collisions that result in excitation and ionization in the sampling region of the low pressure plasma. The glow discharge sources operate at pressures of between 0.5 and 10 Torr (in the presence of inert gases like helium or argon) and at powers of less than 100 Watts. The sustaining power could be in the form of direct current, radio frequency or microwave regions of the electromagnetic spectrum. Atomic emission analysis provides information about the elements present in the composition residing in the sampling region, while mass spectrometry, which may be performed simultaneously with the atomic emission analysis or separately, provides information about both the elements present in the composition and the molecular species present in the composition that resides in the sampling region. Real-time particle sizing through light scattering methods such as a low-power laser scattering device can also be readily achieved. Real-time analysis of the gaseous components that entrain the particles delivered by the conduit to the momentum separator can also be readily achieved by the apparatus and methods of the present invention.
Particle beam interfaces such as momentum separators are an efficient means of introducing particulate matter through xe2x80x9cvacuumxe2x80x9d action (two roughing pumps). A momentum separator has a particle exit port connected in communication with the particle input port of the glow discharge unit or other device for producing a low pressure plasma. The momentum separator has a particle entrance port connected in communication with the particle exit port.
The conduit is a hollow tube that has an entrance opening on one end and an exit opening on the opposite end. The exit opening of the conduit is connected to the entrance port of the momentum separator. The conduit functions to provide a path for directing and transporting to the momentum separator, the gaseous matter that contains the entrained particulate matter that is to be analyzed. One embodiment of the conduit takes the form of a sniffer that includes a restricted flow portion.
It is believed that this general approach holds much promise as process monitoring needs and federal regulations require further chemical information beyond the present particle size standards. The use of vacuum sample introduction, low plasma powers in the glow discharge unit, and small size (less than a shoebox), suggest applications wherein introduction of discrete (collected) samples or continuous remote monitoring can be easily envisioned. The most obvious application of any such methods would be in direct, remote exhaust stack monitoring.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and, together with the description, serve to explain the principles of the invention.